Automobiles are steered by a system of gears and linkages that transmit the turning motion of the steering wheel to the front wheels. As automobile designs shift weight to the front wheels to improve riding comfort and vehicle handling, more effort is needed to turn the front wheels and provide sufficient torque to overcome the friction that exists between the front wheels and the road.
Power steering systems are designed to reduce steering effort and improve maneuverability. Some vehicles use engine driven hydraulics to amplify the torque applied by the steering wheel to the front wheels. A mechanically-driven or an electrically-driven pump maintains a hydraulic fluid, such as oil, under pressure. The rotation of the steering wheel actuates a valve, which supplies or drains fluid to a power cylinder, which reduces the steering effort needed to turn the wheels.
Some vehicles mechanically couple an electric motor to the steering shaft through steering gears. Variable torque assist levels can be realized when speed sensitive controllers alter the required torque to maneuver a vehicle based on vehicle speed. Such voltage mode controller systems are typically controlled by Pulse Width Modulation (PWM) circuits that drive gate circuits and Field Effect Transistor (FET) switches. However, nonlinearities in the circuit components (i.e. gates and FETs) result in voltage amplitude ripple in the controller, particularly when operating in an open loop mode. Moreover, at low values of PWM modulation index, these voltage amplitude variations result in noticeably unacceptable torque ripple components. For example, at low amplitude values of the modulating waveform, the PWM pulses are narrow and any nonlinearity in the gate drive or FETs will contribute a substantial amount of error. In particular, if the PWM pulse is 1% of full value, and the switching frequency is 20 kHz, the pulse width is 500 ns. A 50 ns nonlinearity would result in a 10% error in the width of the voltage provided by the motor controller, whereas a 50 ns nonlinearity on a PWM pulse at 10% or more of full value would result in a 1% or less error in the width of the voltage provided by the motor controller, which is much less noticeable.
To date, the only torque ripple component which remains a significant problem is associated with first-order power stage nonlinearities (called 1-per rev component). Other torque ripple components (e.g. second or third order nonlinearities called 2-per rev or 3-per rev components respectively) related to switch dead time and phase resistance imbalance have been mitigated to the point where they are no longer a significant contributor to the torque ripple problem.
Therefore, it is desirable to provide a system and method of reducing torque ripple in a motor controller that overcomes most, if not all, of the preceding problems.